The possibility of constructing optoelectronic devices, full-color displays, and optical sensors based on silicon has generated tremendous interest in the preparation and characterization of light emitting silicon nanoparticles. Because the particles' luminescence properties are size-dependent, multiple colors can be produced using a single material. These particles also have exciting potential applications as fluorescent tags for biological imaging, as has been proposed for II-VI compound semiconductor nanoparticles. Chan et al., Science 281:2016-2018 (1998); Bruchez et al., Science 281:2013-2015 (1998); Lacost et al., Proc. Natl. Acad. Sci., USA 97:9461-9466 (2000). They can be brighter and much more stable to photobleaching than the organic dyes used in these applications, and they also have much broader excitation spectra, so that emission at multiple wavelengths (from particles of different sizes) can be excited by a single source. There are established methods for preparation of luminescent porous silicon (Lockwood (ed.), Light Emission in Silicon From Physics to Devices (1998)), and aerosol synthesis of macroscopic quantities of non-luminescent silicon nanoparticles has been known for over 20 years. Cannon et al., J. Am. Ceramic Soc. 65:324-335 (1982). However, there are no reported methods for producing macroscopic quantities (i.e. more than a few milligrams) of luminescent silicon nanoparticles that are free from a substrate. Soon after the initial discovery of photoluminescence from porous silicon (Canham, Appl. Phys. Lett. 57:1046-1048 (1990)), Brus and co-workers published a series of papers (Wilson et al., Science 262:1242-1244 (1993); Littau et al., J. Phys. Chem. 97:1224-1230 (1993); Brus, J. Phys. Chem. 98:3575-3581 (1994); Brus et al., J. Am. Chem. Soc. 117:2915-2922 (1995)) in which they prepared silicon nanoparticles by high temperature decomposition of disilane. These studies were instrumental in building an understanding of photoluminescence mechanisms in silicon nanostructures. However, in their actual particle synthesis experiments, they collected less than 10 mg of particles per 24 hour day of reactor operation. Littau et al., J. Phys. Chem. 97:1224-1230 (1993). Small quantities of luminescent silicon nanoparticles have been prepared by laser vaporization controlled condensation (LVCC). Carlisle et al., Chem. Phys. Lett. 326:335-340 (2000); Carlisle et al., J. Electon Spectrosc. 114-116:229-234 (2001).
Recently, Korgel and co-workers prepared brightly luminescent silicon nanoparticles in supercritical organic solvents at high temperature (500° C.) and pressure (345 bar). Ding et al., Science 296:1293-1297 (2002); English et al., Nano. Lett. 2:681-685 (2002); Holmes et al., J. Am. Chem. Soc. 123:3743-3748 (2001). Again, they have produced beautiful and well-characterized particles, but in quite small quantities. In their first report, 0.2 ml per batch of 250-500 mM diphenylsilane was converted to silicon nanoparticles with a yield of 0.5% to 5%, which corresponds to 0.07 to 1.4 mg of Si nanoparticles per batch. Brightly luminescent silicon nanoparticles have been prepared by dislodging them from luminescent porous silicon wafers prepared electrochemically (Nayfeh et al., Appl. Phys. Lett. 78:1131-1133 (2001); Nayfeh et al., Appl. Phys. Lett. 77:4086-4088 (2000); Belomoin et al., Appl. Phys. Lett. 80:841-843 (2002)), and this has also generated tremendous technological and scientific interest. However, this method also generates small quantities of silicon nanocrystals, and the emitting nanocrystals may be embedded in larger porous silicon particles.
The first solution phase synthesis of silicon nanoparticles was presented by Heath (Science 258:1131-1133 (1992)). More recently Kauzlarich and co-workers have demonstrated several procedures (Baldwin et al., Chem. Comm. 17:1822-1823 (2002); Bley et al., J. Am. Chem. Soc. 118:12461-12462 (1996); Liu et al., Mat. Sci. Eng. B. B96:72-75 (2002); Mayeri et al., Chem. Mat. 13:765-770 (2001)) for producing silicon nanoparticles with a variety of surface terminations at mild conditions in solution using reactive Zintl salts. They are able to produce larger quantities of particles than the methods described in the previous paragraph. In some cases, they have shown blue-UV photoluminescence from these particles, but appear not to have observed the orange to red PL characteristic of porous silicon and most other nanoparticle preparation methods, including that presented here. Solid phase reactions have also been used to produce larger quantities of silicon nanoparticles, but apparently with much lower PL efficiency than the particles measured in the previous paragraph. Lam et al. produced silicon nanoparticles by the reaction of graphite with silicon dioxide (SiO2) in a ball mill (J. Crystal Growth 220:466-470 (2000). A wide range of particle sizes were produced, but some PL was observed after ball milling for 7 to 10 days. Ostraat and co-workers have prepared oxide-capped silicon nanoparticles via vapor phase decomposition of highly diluted SiH4 in nitrogen followed immediately by surface oxidation. Ostraat et al., J. Electrochem. Soc. 148:G265-G270 (2001); Ostraat et al., Appl. Phys. Lett. 79:433-435 (2001).
CO2 laser pyrolysis of silane (SiH4) is an effective method of producing gram-scale quantities of silicon nanoparticles, as first shown more than 20 years ago. Cannon et al., J. Am. Ceramic Soc. 65:324-335 (1982). It produces high-purity loosely agglomerated particles with controlled primary particle size and size distribution. Moreover, it is a continuous process that permits reasonable production rates. While several groups have synthesized silicon particles with this and similar methods, the resulting particles showed little or no visible photoluminescence. Borsella et al., Mat. Sci. Eng. B. B79:55-62 (2001); Borsella et al., J. Mat. Sci. Lett. 16:221-223 (1997); Botti et al., J. Appl. Phys. 88:3396-3401 (2000). An exception to this is the work of Huisken and coworkers, who use pulsed CO2 laser pyrolysis of silane, which yields luminescent particles, but in very small quantities. They have also studied the effect of particle aging in air and surface etching with HF on the photoluminescence spectrum. Ehbrecht et al., Phys. Rev. B. 56:6958-6964 (1997); Huisken et al., Appl. Phys. Lett. 74:3776 (1999); Ledoux et al., Mater. Sci. Eng. C19:215-218 (2002); Ledoux et al., Appl. Phys. Lett. 80:4834-4836 (2002); Ledoux. et al., Appl. Phys. Lett. 79:4028-4030 (2001).
In order to show efficient visible photoluminescence (PL), it is believed that silicon nanoparticles must be smaller than 5 nm, and their surface must be ‘properly passivated’ such that there are no non-radiative recombination sites on it. The mechanism(s) of photoluminescence in silicon nanocrystals and the effect of surface passivation on light emission from them remain topics of active research and debate. The size of silicon nanoparticles can be reduced by etching them in mixtures of hydrofluoric acid (HF) and nitric acid (HNO3) (Seraphin et al., J. Mater. Res. 12:3386-3392 (1997)) as well as by aging them in air then removing the resulting oxide with HF. Ledoux. et al., Appl. Phys. Lett. 79:4028-4030 (2001).
The present invention is directed at overcoming these and other deficiencies in the art.